1. Field of the Invention
The present invention relates to a photo-responsive liquid crystal composition, and an information-recording medium and an information-recording process making use of this liquid crystal composition.
2. Description of the Background Art
In recent years, attention has been attracted to the so-called supermolecule materials formed by a phenomenon that self-coalescence or self-organization of molecules is caused by a non-covalent bonding intermolecular interaction such as hydrogen bonding. In particular, a substance called xe2x80x9cliquid crystal physical gelxe2x80x9d contains, a liquid crystal that is a functional fluid as a solvent, and has both optical properties and electrical properties as a liquid crystal, and has a nature as a soft solid that is called physical gel, and hence attracts one""s attention as novel functional materials.
The physical gel is such that is gelled by causing molecules to coalesce in the form of fibers by an intermolecular interaction such as hydrogen bonding to form a network and taking a solvent therein. In addition, the microscopic intermolecular interaction of this physical gel can be changed according to external stimulation or external environment. As a result, the gel has a possibility that the coalesced state or organized form of the gel may be changed. These changes finally come to bring macroscopic physical changes in the physical gel. Accordingly, the liquid crystal physical gel capable of controlling the coalesced form or organized form thereof by external stimulation is expected as a really novel polyfunctional material or highly functional material.
Temperature, electric field, light, etc. are considered as examples of external stimulation for controlling the functions related to the coalesced form or organized form of the liquid crystal physical gel. In particular, stimulation by light is extremely useful in that:
(1) stimulation can be selectively given to a specified part, particularly, a minute region, for example, about several hundred nanometers, at the least,
(2) the intensity of stimulation can be easily controlled, and
(3) stimulation can be given in a state brought into no contact with the material.
With respect to such liquid crystal physical gel, it has been reported to use, for example, an azobenzene compound as a gelling agent (Chem. Mater., 12, 3667 (2000); J. Mater. Chem., 11, 1339 (2001)).
An azobenzene compound itself is a photochromic compound undergoing isomerization from a trans-form to a cis-form by irradiation of ultraviolet light and isomerization from the cis-form to the trans-form by irradiation of visible light or heating.
However, there has been proposed to date nothing about the utilisation of fact that the photo-responsibility of a photochromic compound such as an azobenzene compound for the purpose of controlling the coalesced form or organized form of the liquid crystal physical gel.
The present inventors have carried out repeated researches as to changes in structure or phase state of a gelling liquid crystal composition, which is obtained by mixing a gelling agent composed of a specific photochromic compound such as an azobenzene compound with a liquid crystalline compound due to molecular coalescence or organization of the composition, which are caused by stimulation by light. As a result, the present invention has been led to completion.
It is the first object of the present invention to provide a photo-responsive liquid crystal composition which is composed of a liquid crystalline compound and a gelling agent consisting of a photochromic compound, may exist in any state of gel of a first phase and gel of a second phase different from the first phase, and may undergo changes in phase state between the gel of the first phase and the gel of the second phase.
The second object of the present invention is to provide an information-recording medium having a recording layer formed of the photo-responsive liquid crystal composition described above.
The third object of the present invention is to provide an information-recording process making use of the information-recording medium described above.
According to the present invention, there is thus provided a photo-responsive liquid crystal composition comprising a liquid crystalline compound and a gelling agent mixed with the liquid crystalline compound to form a gelling mixture, wherein
the gelling agent consists of a photochromic compound having a group capable of forming an intermolecular hydrogen bond in its molecule as well as a photo-responding site due to at least one of an azobenzene structure, a spiropyran structure, a fulgide structure and a diarylethene structure, and wherein
the composition may exist in any state of gel of a first phase and gel of a second phase different from the first phase and may undergo changes in phase state between the gel of the first phase and the gel of the second phase.
In the photo-responsive liquid crystal composition according to the present invention, the gelling agent may preferably be an azobenzene compound represented by the following general formula (1): 
wherein X is a divalent organic group, and Y is a monovalent organic group.
According to the present invention, there is also provided an information-recording medium comprising a recording layer formed by the photo-responsive liquid crystal composition described above.
According to the present invention, there is further provided an information-recording process making use of the information-recording medium described above, which comprises the step of irradiating a selected region or regions of the recording layer of the information-recording medium with light according to information to be recorded, thereby forming an information-recorded portion of the gel of the second phase of a pattern according to the information in a matrix of the gel of the first phase, and fixing the information to the recording layer.
According to the present invention, there is still further provided an information-recording process, comprising the steps of irradiating a recording layer formed by a photo-responsive liquid crystal composition composed of a nematic liquid crystalline compound and the gelling agent described above with ultraviolet light according to information to be recorded in a state that the phase of the recording layer is nematic gel, thereby changing a selected region or regions of the nematic phase to a cholesteric phase of a sol state, irradiating the cholesteric phase of the sol state with visible light, thereby changing the cholesteric phase of the sol state to a cholesteric phase of a gel state to form an information-recorded portion of the cholesteric phase of the gel state of a pattern according to the information in a matrix of the nematic phase of the gel state, and fixing the information to the recording layer.
The process may further comprise the step of heating the recording layer having the cholesteric phase of the gel state, thereby changing the whole phase of the recording layer to nematic gel to erase the fixed information.
In such a photo-responsive liquid crystal composition as described above, the gelling agent mixed with the liquid crystalline compound is a photochromic compound having a group capable of forming an intermolecular hydrogen bond in its molecule as well as a specific photo-responding site, whereby the chirality of the gelling agent is reflected on the liquid crystalline compound according to the change in gelling ability due to photo-isomerization of the gelling agent, i.e., the change in solubility thereof in the liquid crystalline compound. As a result, a gelled nematic liquid crystal, for example, may be changed to a cholesteric phase of a sol state, and this cholesteric phase of the sol state may be further changed to a cholesteric phase of a gel state. The photo-responsive liquid crystal composition may thereby exist in any state of gel of a first phase and gel of a second phase different from each other and may undergo changes in state between the gel of the first phase and the gel of the second phase.
According to the composition, a state that the gel of the second phase has stably coexisted in a selected region or regions of a matrix of the gel of the first phase may be formed, so that information by proper light or image information can be recorded by utilizing the characteristics.
Further, the recording layer is heated, whereby the whole recording layer may be changed to an isotropic phase, thereby erasing the information recorded in the recording layer. In addition, the recording layer can be used in recording of new information.